This invention relates to cabled conductors comprising superconducting ceramics and to a method for using them. Superconductors have gained increasing attention for having the potential to improve the efficiency of electric power and magnetics applications. When held below the critical temperature, current and magnetic field which defines its superconducting state, every superconductor is able to carry DC currents with very little energy loss. Equally important for most conductor applications is the ability of superconductors to carry very high currents, with current densities of thousands of times that of conventional copper conductors. Certain of the ceramic superconductors have the potential to maintain these properties at cryogenic temperatures in the general range of the boiling point of nitrogen. However, several different types of resistive losses can occur in ceramic superconductors. Unlike low transition temperature superconductors, which transition abruptly between the high performance superconducting and low performance normal, i.e., resistive, states if one of their critical values is exceeded, the ceramic superconductors do not. Consequently, for ceramic superconductors, a measurement procedure, known as the offset criterion or the electric field (E) criterion, described, for example, in High T.sub.c superconductors and critical current measurement, Cryogenics, Vol 30, pp 667-677 (August 1990), and Offset criterion for determining super-conductor critical current, Appl. Phys. Lett. 55(9) (28 August 1989), both of which are herein incorporated in their entirety by reference, is often used to establish the the critical current value, i.e. the current value at which the transition between superconducting and non-superconducting states is considered to occur in ceramic superconductors. This procedure defines the critical current density (J.sub.c) as the current density (J) where the tangent to the electric field (E) vs J curve (for a specified temperature and magnetic field level) at a given electric field level, such as 1 .mu.V/cm, extrapolates to zero electric field. For brevity, the procedure is typically referenced by the value of its electric field criterion. In the transition regime, resistive losses gradually increase to their non-superconducting values. Above the critical current value, the transition regime is known as the flux flow state.
In time varying magnetic fields or currents, all conductors, including ceramic superconductors in their superconducting and flux flow states, have losses which may include hysteresis and various types of coupling losses. These vary with frequency, AC and DC current amplitude and conductor geometry. Periodic multifilamentary, multistranded structures with short repeat lengths have been shown to minimize AC losses for low transition temperature superconductors, as described, for example, in "Superconducting Magnets" by Martin Wilson (1983,1990), pp 197, 307-309, which is herein incorporated by reference, and in conventional copper conductors. Both low transition temperature supercondutors and conventional copper conductors are commonly fabricated into well-known cabled forms, such as Litz cables, Rutherford cables (a type of Litz cable), Roebel cables, or braids for use in time-varying magnetic fields or currents. U.S. Pat. No. 3,764,725 issue Oct. 9, 1973 to Kafka, U.S. Pat. No. 4,857,675 issued Aug. 15, 1989 to Marancik et al, U.S. Pat. No. 1,144,252 issued Jun. 22, 1915, all of which are herein incorporated in their entirety by reference, teach the use of braided Litz, Rutherford, and Roebel cabled forms, respectively. Typically, for low transition temperature superconductors, techniques such as twisting and bending are used to transpose both the filaments in a conductor strand (to minimize filament coupling losses) and the strands in a multistrand conductor (to minimize strand coupling losses) about their central longitudinal axes.
It has been proposed that similar periodic geometries would minimize certain types of AC losses in ceramic superconductors so they would be desirable for any electrical or magnetic application involving time-varying currents or magnetic fields. For example, U.S. Pat. No. 5,038,127 issued Aug. 6, 1991 to Dersch, describes two periodic arrangements of coated conductors intended to reduce eddy current losses.
However, ceramic superconductors have physical limitations, namely anisotropy and low critical strain values, which typically create very high resistive losses in long lengths of high winding density, tightly cabled conductor. Critical strain is rarely an issue for conventional metal cables. Anisotropy is not a design constraint for cables made of either conventional metal or low transition temperature superconductors.
The superconducting ceramics which have shown greatest promise for electrical and magnetic applications at relatively high temperatures are anisotropic superconducting compounds which require texturing in order to optimize their current-carrying capacity. The current-carrying capacity of any composite containing one of these materials depends significantly on the degree of crystallographic alignment, or "texturing", and intergrain bonding of the superconductor grains induced during the composite manufacturing operation. Suitable texturing methods, all of which are well known in the art, include, for example, various heat treatments to obtain reaction-induced texturing, various deformations to obtain deformation-induced texturing, growth on a textured substrate material, and magnetic alignment. For example, known techniques for texturing the two-layer and three-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors are described in Tenbrink, Wilhelm, Heine and Krauth, Development of Technical High-Tc Superconductor Wires and Tapes, Paper MF-1, Applied Superconductivity Conference, Chicago Aug. 23-28,1992), and Motowidlo, Galinski, Hoehn, Jr. and Haldar, Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors, paper presented at Materials research Society Meeting, Apr. 12-15, 1993, Kase et al, IEEE Trans. Mag. 27(2), 1254(1991), and U.S. Ser. No. 08/041,822 filed Sep. 8, 1994, entitled "Torsional Texturing of Oxide Superconducting Articles", all of which are herein incorporated in their entirety by reference. Some techniques for forming and texturing the yttrium family of oxide superconductors are described, for example, in L. J. Masur et al, Physica C 230 (1994) 274-282, M. Fukutomi et al, Physica C 219 (1998) 333-339, and V. Chakrapani, D. Balkin, P. McGinn, Applied Superconductivity, Vol. 1, No. 1/2,(1993), pp. 71-80, all of which are herein incorporated in their entirety by reference. Suitable final heat treatment processes for BSCCO 2223, which are believed to contribute to intergrain bonding via partial melting and crack healing are described, for example, in copending applications U.S. Ser. No. 08/041,822 filed Apr. 1, 1993 and entitled "Improved Processing for Oxide Superconductors", U.S. Ser. No. 08/198,912, filed Feb. 17, 1994 and also entitled "Improved Processing for Oxide Superconductors", and in U.S. Ser. No. 08/553,184, filed Nov. 7, 1995 and entitled "Processing of Oxide Superconducting Cables", all of which are herein incorporated in their entirety by reference.
The desirable crystallographic structure of well-textured ceramic superconductors causes them to have extremely anisotropic current carrying capability, with the highest current flowing in the directions lying in the crystallographic plane containing the a and b direction vectors of the aligned grains, or in other words, orthogonal to the c direction of each grain. Critical current and critical magnetic field may be as much as an order of magnitude lower in a "bad" direction of a well-aligned oxide superconductor than in a "good" direction lying in the crystallographic plane containing the a and b direction vectors of the grains. It is conventional to refer to the set of directions in a crystallographic plane by the vector which is orthogonal to all of them. The c direction is commonly referred to as the preferred direction of the superconductor, because it uniquely defines the set of directions lying in the "good" plane of the material. Any design which relies to a significant degree on current transport in the "bad" direction of the material, such as the designs disclosed in Dersch, cited above, or the "core-wrap" designs discussed below, will be constrained to operate well-below the critical current carrying capacity of the "good" direction, or will have significant resistive losses.
Moreover, ceramic superconductors are typically fragile, brittle, granular compounds, which cannot by themselves be easily processed into traditional conductor forms. Stranded conductors such as wires and tapes are generally made by forming composites containing one or more filaments of the ceramic superconductor in intimate contact with a matrix material, typically a noble metal such as silver or a silver alloy, to form more ductile conductors. Known methods for forming composite wires and tapes include the powder-in-tube (PIT) method, using oxide or metallic precursors, a physical film forming method such as sputtering or ion beam assisted deposition (IBAD), a chemical film forming method such as chemical vapor deposition (CVD), or the like. For example, multifilamentary wires and tapes made by the PIT process may be used. The general PIT process is described, for example, in U.S. Pat. Nos. 4,826,808, and 5,189,009 to Yurek et al. and L. J. Masur et al, Physica C 230 (1994) 274-282, which teach the use of a metal alloy precursor having the same metal content as the desired superconducting oxide, and in C. H. Rosner, M. S. Walker, P. Haldar, and L. R. Motowidlo, "Status of HTS superconductors: Progress in improving transport critical current densities in HTS Bi-2223 tapes and coils" (presented at conference `Critical Currents in High Tc Superconductors`, Vienna, Austria, April, 1992) and K. Sandhage, G. N. Riley Jr.,. and W. L. Carter, "Critical Issues in the OPIT Processing of High Jc BSCCO Superconductors", Journal of Metals, 43,(1991) 21-25, which teach the use of either a mixture of powders of the oxide components of the superconductor or of a powder having the nominal composition of the superconductor, all of which are herein incorporated in their entirety by reference. An ion-beam-assisted deposition process is described in M. Fukutomi et al, Physica C 219 (1994) 333-339, which is herein incorporated in its entirety by reference. But these composite strands are still brittle by the standards of conventional conductors, and cannot be twisted or otherwise bent to a tight radial arc without a reduction of available current density due to microcracking which disrupts the current paths through the filaments. FIG. 3 shows the normalized critical current, I.sub.c /I.sub.co (the critical current at the applied strain divided by the critical current at zero strain) as a function of applied strain. For most ceramic superconducting composites, the critical current, I.sub.c, is independent of the amount of tensile strain placed on the composite until the strain reaches a threshold value, commonly referred to as the critical strain of the material. Above that threshold, the critical current value decreases asymptotically with increasing tensile strain, as shown in FIG. 3, due to formation of localized microcracks in the material. In typical superconducting oxide filaments, for example, available current density begins to drop at bends which create only a fraction of a percent of strain, depending on the filament cross-section and material. To make high packing factor cables from these materials would require bend strains far in excess of the tolerances of typical superconducting ceramic filaments. Although methods for repairing microcracking are known, if the local tensile strain is much greater than the critical strain value, as it is likely to be due to the bending and compression forces exerted during the cabling operations, micro-crack formation can occur to such an extent that significant healing during a later thermomechanical processing step becomes impossible. When this happens, portions of the filament revert to partially superconducting or totally resistive states, with high power losses per unit area.
Thus, in designing superconducting ceramic conductors suitable for use in time-varying magnetic fields or currents, the loss reductions achieved by periodic transposition must be balanced against the increased resistive losses from straining the superconductor or from orienting it in its bad direction in order to acheive transposition. Various "core-wrap" designs for high temperature superconducting cable have been proposed in which the conductor strands are helically wound in a gentle arc around a large central cooling pipe or other supporting structure in order to minimize the strand bending radii. However the packing factor of these designs is unacceptably low for most commercial purposes such as magnets or coils. Together with total power consumption, which includes both AC losses and resistive losses, packing factor determines the suitability of a cabled conductor for a given application. For example, a 0.010" thick strand wrapped to a bend strain of 1% around a hollow central core would require a wrap radius of 0.5", so the total packing factor of the finished cable and core would be on the order of 2-3%, not the 75% or more desired for commercial applications. In addition, if the filaments are textured while flat in accordance with conventional practices, it is not possible to transpose them through a uniform 360 degree arc without orienting roughly half of their current carrying capacity in the "bad" direction relative to the applied magnetic field. If the current flow through this "bad" portion exceeds its critical value, there will be significant resistive losses. Because the radius of this class of cabled conductor designs must be so large to offset its mechanical and electrical limitations, and so much of the cable is oriented in directions where its current carrying capacity is substantially reduced, these designs have found limited commercial acceptance despite the potential advantages of superconducting ceramic cables. Designs in which the superconducting ceramic is strained well past its critical limit have not been proposed.
A practical design for a low loss, high winding density cabled conductor specifically adapted to the strengths and limitations of superconducting ceramic composites has yet to be provided.
Thus, an object of this invention is to provide a cabled conductor design using superconducting ceramic composites which can provide the combined advantages of low overall power consumption and a superior packing factor at a given current density, operating temperature and magnetic field than existing designs.
Another object of this invention is to provide improved cabled conductor designs with low resistive losses and high current carrying capacity in comparison to helically wound superconducting ceramic or conventional copper and silver cabled conductors.